In the manufacture of cigarettes, tobacco leaf is processed to separate the stems from the lamina. The lamina are shredded and formed into cigarettes or other smoking articles. The stems are not successfully utilizable as such in cigarette making because of their relatively large diameter, their hard nature and poor burning properties.
Nevertheless, tobacco stem material constitutes a substantial proportion of the leaf, usually about 20 to 25% of the weight thereof, and contains tar, nicotine and other materials common to the lamina. In the past, therefore, efforts have been made to process the stem material for use in smoking articles but such procedures have generally been unsatisfactory, unsuccessful or inefficient.
One prior art procedure which has been adopted, involves passing the stems, usually after moistening to about 30 wt.% moisture, between rollers which act to crush the stems into sheet material, and cutting the sheet material into shreds for mixing with shredded lamina from which the smoking article is made. The product of this operation is commonly termed "cut rolled stem" (CRS). Cut rolled stem suffers from the drawbacks that it has only limited filling power, i.e., it has a limited ability to fill a cigarette tube, and hence more material is required to be present in the cigarette tube to achieve the same hardness of cigarette than for a higher filling power material.
A variation of this prior art procedure involves soaking and fast drying of the cut rolled stem, which produces a product commonly known as "enhanced cut rolled stem" which has an improved filling power as compared with the cut rolled stem.
Another prior art procedure is described in U.S. Pat. No. 4,094,323 assigned to American Brands Inc. wherein the stem material first is conditioned at a temperature of 115.degree. to 170.degree. C. under a superatmospheric pressure and thereafter is fiberized and expanded using a pressurized shredder comprising closely-spaced counter-rotating ribbed plates at an elevated temperature of about 115.degree. to 170.degree. C. This procedure produces a fiberized and expanded stem material having a substantially improved filling power when compared with cut rolled and enhanced cut rolled stem.
However, this latter prior art procedure also produces a considerable proportion of fine particulate material or "dust", as determined by passing through an 18-mesh sieve, usually about 30 wt.%, which is less suitable for use directly in cigarette making. The particulate material may be separated from the fiberized expanded stem prior to utilization of the latter and may be used to form reconstituted tobacco sheet, but such a procedure is not always available and also means additional processing. The properties ascribed to the fiberized stem material in U.S. Pat. No. 4,094,323 are determined after separation of the fine particulate material.
In addition, while the fiberized and expanded stem material have a substantially improved filling power, when compared with CRS or enhanced CRS, other properties of the product are less satisfactory from the point of view of utility. The burn rate of the material is substantially increased with respect to CRS and enhanced CRS, meaning that, while a lesser combined weight of shredded tobacco lamina and stem may be used in the cigarette for the same overall hardness as a blend of shredded tobacco lamina and CRS, nevertheless, the increase in burning rate of the cigarette which results from the lesser overall amount of tobacco and the increased burning rate of the fiberized and expanded stem material is detrimental.
Furthermore, the pressure drop which results along a cigarette made therefrom is substantially greater than for a cigarette made from enhanced CRS which in turn is greater than for a cigarette made from CRS, under the same cigarette conditions. The pressure drop along the cigarette relates to the ability of the smoker to draw smoke from the cigarette into his mouth, and lower values are generally considered more satisfactory than higher values.
The problem to which the present invention is directed is the production of a shredded stem material having a decreased burn rate compared with CRS, enhanced CRS and the fiberized enhanced stem and an improved filling power when compared with CRS and enhanced CRS while not significantly adversely affecting the pressure drop characteristics when compared with CRS, while at the same time avoiding the production of large amounts of fine particulate matter during the shredded stem formation.
In accordance with one embodiment of the present invention, there is provided a novel shredded tobacco stem material in substantially fibrous form which is characterized by a burn rate of less than about 70 mg/min, preferably about 40 to about 60 mg/min.
The term "burn rate" as used herein with reference to the product of the invention and other processed stem material refers to the rate of burning of the material when formed into a cigarette having a diameter of 7.95 mm, a moisture level of 12.5 wt.% and a density of 0.215 g/cc wrapped in non-porous phosphate cigarette paper.
When formed into a cigarette, the novel stem material also produces a carbon monoxide content in the smoke of 0.47 to 1.14 mg per puff, as determined by Canadian Standard smoking procedures leaving a 30 mm butt.
The novel shredded tobacco stem material also usually exhibits specific pressure drop and filling power characteristics when formed into a cigarette. The novel shredded tobacco material usually exhibits a pressure drop through the tobacco portion of the cigarette of 2.5 to 3.5 cm of water at a flow rate of 17.5 ml/sec, a cigarette density of 0.215 g/cc and a cigarette length of 85 mm. The filling power of such stem material usually is 5.2 to 6.0 g/cc.
The novel shredded stem material is formed by a novel shredding method, which may also be used for tobacco stalk material and cigarette making machine winnowings, which forms a second embodiment of the invention. In accordance therewith, a mass of tobacco stem, stalk or winnowings is treated with water to uniformly distribute water throughout the mass and to thoroughly soak the tobacco stem, stalk or winnowings within the mass to provide an overall moisture content of about 30 to about 60% by weight. The soaked stem, stalk or winnowings is mechanically fiberized between closely-spaced fiberizing surfaces at atmospheric pressure to form shredded tobacco stem, stalk or winnowings in substantially fibrous form. The shredded material is dried to any desired moisture content.
That latter procedure, while producing a shredded stem product of novel characteristics, also produces considerably less particulate material, typically about 20 wt.% thereof passes through an 18-mesh screen, than the prior art procedure of U.S. Pat. No. 4,094,323. This significantly decreased proportion of fine particles permits the whole of the processed material to be utilized without the necessity for the separation of the small particles, since the quantity and characteristics of the particulate material present does not significantly adversely affect the properties of the shredded material and the properties specified herein are those for the whole of the shredded material. It has been found, however, that properties of the shredded material, in particulate burn rate and filling power, are further improved, if the fine particulate material is removed from the shredded material. The necessity to discard or further treat a significant proportion of the shredded material, such as occurs in the prior art procedure of U.S. Pat. No. 4,094,323, is eliminated.